An interferometric measuring device in which such a fiber probe is used is described in German Patent No. DE 102 44 553. That document describes an interferometric measuring device for detecting the shape, the roughness, or the distance from the surface of a measured object using a modulation interferometer, to which a short-coherent radiation is supplied by a radiation source and which has a first beam splitter for splitting the supplied radiation into a first partial beam conducted via a first arm and a second partial beam conducted via a second arm, the light phase or light frequency of one of the partial beams being shifted with respect to the other by a modulation device and this beam passing through a delay segment, the two partial beams being subsequently combined in a further beam splitter of the modulation interferometer, having a measuring probe spatially separated from the modulation interferometer and connected or connectable thereto via a fiber optic device in which the combined partial beams are split into a measuring beam conducted to the surface by a fiber optic probe device having an oblique object-side exit surface and a reference beam and in which the measuring beam (r1(t)) reflected on the surface and the reference beam (r2(t)) reflected on a reference plane are superimposed, and having a receiving device and an analyzing unit for converting the radiation supplied thereto into electrical signals and for analyzing the signals on the basis of a phase difference. The angle of inclination (y) of the exit surface is at least 46° with respect to the normal to the optical probe axis.
According to FIGS. 5 and 6 shown in the document, the oblique exit surface of an object-side optical fiber probe unit projects over a tube-shaped receptacle in such a way that the outstanding optical fiber probe unit, i.e., the fiber end piece, can be introduced into narrow cavities, for example for optical, in particular interferometric, measuring functions.
The depth of the cavity to be tested is limited by the length of the fiber end piece. In optical fiber probe units used today, the length of the fiber end piece for a typical fiber diameter of 20 μm is approximately 2 mm. Due to the mechanical sensitivity of the fiber end piece and its tendency to vibrate, with the corresponding negative effects on the measuring accuracy, the fiber end piece cannot be made substantially longer. This limits the use of the interferometric measuring unit to testing cavities having a relatively shallow depth.
It is therefore an object of the present invention to provide an optical fiber probe which enables high measuring-accuracy testing of cavities having small diameters and great depths with the simplest possible handling.